This invention relates to a method and apparatus for treating tanks requiring both internal coating and pressure testing.
It has application to many items of manufacture but has particular application in the field of serially produced mains pressure water heaters which include a vitreous enamel internally coated steel tank. For convenience the invention will be described in relation to the example of the vitreous enamel lined steel tank, its wider applicability being thus made apparent.
The invention is illustrated in relation to a process of treating the internal surface of a water heater tank with a vitreous enamel coating material in liquid form, hereinafter referred to as slip, involving the filling of the substantially assembled tank with slip, testing the structural integrity of the thus-far assembled tank by application of a stipulated hydrostatic internal pressure to the tank, that pressure being transmitted by the slip medium itself. Further steps comprise emptying the tank of slip in a manner effective in controlling the amount of the residual wet slip coating left adhering to the tank walls, improving the uniformity of the thickness of the wet coat, drying the coat and furnace heating to melt the dried slip to form a glazed enamel corrosion protection layer strongly resistant to the corrosive effect of prolonged hot water contact.
In order to highlight differences between the present invention and what has been previously done, it is explained that manufacturing steps for mains pressure electrically heated storage water tanks comprise formation of at least three components. These are a cylindrical body and two curved ends. Under a procedure to be discontinued by the adoption of the method described in relation to the present invention, the body and top end are assembled together--the top end concave to the tank interior and then welded circumferentially around the outside of the junction of the two assembled parts.
The welded top and body component are then internally spray coated with slip and the bottom is also sprayed with slip on the convex face. The parts are then dried at a rate sufficiently slow to avoid spalling-off or any other deterioration of the coating during subsequent operations. After drying the separate parts are furnace heated to melt the solids content of the slip, the result being that the parts are coated by a glazed enamel coating.
The bottom is then assembled to the bottom of the tank, convex side inward, and circumferentially welded around the junction.
A disadvantage of this known technique is that great skill and control is required in performing the welding process in order that the glazed surface is not cracked or crazed by the heat of welding and that the weld achieves full metal fusion free of glass inclusion.
An alternative method of coating the interior of hollow vessels with vitreous enamel is known and is described in British patent 1 078 903. In this patent a process is described whereby a cylindrical tank having both curved ends already welded to the body has an opening in the centre of each end enabling liquiform enamel slip to be drawn into the lower end by vacuum applied to the upper end. After filling with enamel the tank is drained, dried and furnace heated. Known tanks for water heating and storage made using this process have, when completed for use, fitted to the bottom opening, a flange which is closed by a bolted-on plate through which pass fittings such as for supporting a heating element or coil, as well as a cold water inlet pipe. This is suited to traditional European practice of using bottom entry hot water storage tanks supported on walls. Elsewhere, preferred practice is to have floor standing hot water storage tanks with side entry and delivery water connections, side entry electrical heating element(s) and relief valve and a top central entry for an elongate anode for augmenting the protection given to the steel by the vitreous enamel coating.
Consumer demand for water heaters often requires a minimum height for the assembled complete heater when plumbed, for reasons of being able to fit the unit in restricted headroom such as under sinks, in cupboards or under houses. The European practice of smaller capacity, higher stored water temperature heaters with bulky flanges top and bottom and with plumbing and electrical entry at the bottom, does not suit certain consumers.